1. Field of the Invention
The present invention relates to a conductive film substrate, a photovoltaic cell having the same, and a method of manufacturing the same, and more particularly, to a conductive film substrate which includes a transparent conductive film made of zinc oxide (ZnO), a photovoltaic cell having the same, and a method of manufacturing the same.
2. Description of Related Art
Photovoltaic cells are a key device for photovoltaic power generation that directly converts solar energy into electrical energy. At present, photovoltaic cells are being applied in a variety of fields, including the supply of electricity to electrical and electronic products, houses and buildings as well as industrial power generation. The most basic structure of photovoltaic cells is a p-n junction diode. Photovoltaic cells are divided into a variety of types, for example, a silicon (Si) photovoltaic cell which uses Si for a light-absorbing layer, a chemical photovoltaic cell which uses CuInSe2 (CIS) or cadmium telluride (CdTe) for the light-absorbing layer, a dye-sensitized photovoltaic cell in which photosensitive dye molecules are adsorbed on the surface of nano particles of a porous film such that electrons are activated when the photosensitive dye molecules absorb visible light, and a tandem photovoltaic cell which has a plurality of amorphous Si layers stacked on one another, depending on the material used in the light-absorbing layer. In addition, photovoltaic cells are also divided into bulk type photovoltaic cells (including single crystalline and polycrystalline types) and thin film type photovoltaic cells (including amorphous and polycrystalline types)
The tandem photovoltaic cell has advantages in that it can increase its open voltage and improve a conversion efficiency of incident light.
FIG. 1 is a schematic cross-sectional view showing the structure of a tandem thin-film photovoltaic cell of the related art.
As shown in FIG. 2, the tandem thin-film photovoltaic cell 110 of the related art generally includes a substrate 111, a transparent conductive film 112, a first p-n junction layer 113, a tunneling p-n junction layer 114, a second p-n junction layer 115 and a back reflector 116. The first p-n junction layer 113 having a predetermined band gap (e.g. Eg=1.9 eV) is disposed on the second p-n junction layer 115 having a smaller band gap (e.g. Eg=1.42 eV), such that the first p-n junction layer 113 absorbs photons, the energy of which is greater than 1.9 eV, and the second p-n junction layer 115 absorbs photons, the energy of which is in the range 1.42 eV<hv<1.9 eV.
Here, the light transmittance, conductivity and light scattering effect of the transparent conductive film 112 are required to be high. In particular, since the light absorptivity of amorphous silicon is low, the surface of the transparent conductive film is textured in order to increase the power generation efficiency of the photovoltaic cell by increasing the path of incident light through light scattering (light trapping effect).
The transparent conductive film of the photovoltaic cell is manufactured using tin oxide (SnO2) or zinc oxide (ZnO). However, the tin oxide is reduced by hydrogen plasma created during a plasma-enhanced chemical vapor deposition (PECVD) process, which is intended for the manufacture of the light-absorbing layer of the thin-film photovoltaic cell, thereby decreasing the transmittance of the transparent conductive film, which is problematic. Although the amorphous silicon has an optical response range from 400 nm to 800 nm, the silicon tandem photovoltaic cell has an optical response range from 400 nm to 1100 nm. Therefore, various researches are being carried out on the transparent conductive film using zinc oxide since the light transmittance of zinc oxide in the near infrared (NIR) range is superior to the light transmittance of tin oxide.
In order to improve the power generation efficiency of the photovoltaic cell, a short circuit current, an open circuit voltage and a fill factor are required to be high. In addition, low-resistance characteristics are required for the transparent conductive film in order to reduce the series resistance of the photovoltaic cell. In particular, the transparent conductive film is required to have high charge mobility.
However, the transparent conductive film of the related art has drawbacks in that its charge mobility and low-resistance characteristics are unsatisfactory.
The information disclosed in the Background of the Invention section is only for the enhancement of understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.